The present invention relates generally to automotive body structural members and, more specifically, relates to techniques for increasing strength and stiffness of automotive body structural members.
In a number of applications, particularly in the automotive industry, it is important to provide high strength structural members at the lowest possible mass. A number of composite materials have been proposed by others in the past for use in forming structural members, including exotic lightweight alloys. In the automotive industry, however, the need for mass reduction without sacrificing strength must be balanced against the cost of the product to the consumer. Thus, there is a need for maintaining or increasing the strength of structural members such as rockers, windshield, pillars, radiator support beams, drive shafts, side impact beams, and bumpers without significantly increasing materials and labor costs.
The reinforcement of motor vehicle structural members through the use of composite materials is known. For example, the present inventor has disclosed a number of metal/plastic composite structures for use in reinforcing motor vehicles components. In U.S. Pat. No. 4,901,500, entitled xe2x80x9cLightweight Composite Beam,xe2x80x9d a reinforcing beam for a vehicle door is disclosed which comprises an open channel-shaped metal member having a longitudinal cavity which is filled with a thermoset or thermoplastic resin-based material. In U.S. Pat. No. 4,908,930 entitled, xe2x80x9cMethod of Making a Torsion Bar,xe2x80x9d a hollow torsion bar reinforced by a mixture of resin with filler is described. The tube is cut to length and charged with a resin-based material.
In U.S. Pat. No. 4,751,249, entitled, xe2x80x9cReinforcement Insert for a Structural Member with Method of Making and Using the Samexe2x80x9d, a precast reinforcement insert for structural members is provided which is formed of a plurality of pellets containing a thermoset resin with a blowing agent. The precast is expanded and cured in place in the ""structural member. In U.S. Pat. No. 4,978,562, entitled, xe2x80x9cComposite Tubular Door Beam Reinforced with a Syntactic Foam Core Localized at the Mid Span of the Tubexe2x80x9d, a composite door beam is described which has a resin-based core that occupies not more than one-third of the bore of a metal tube.
In addition to the present inventor""s own prior work, a number of metal laminates constructions are known in which flat metal plates are bonded together by an intervening layer of resin. It is also known to form a metal laminate sheet for use as a floor panel member which comprises, a pair of flat metal sheets having an intervening layer of asphalt or elastic polymer.
Although filling sections with plastic foam does significantly increase section stiffness (at least when high-density foams are utilized), they also increase mass and thus part weight, which, as stated, is an undesirable feature in automotive applications. Moreover, although increasing the metal gauge of a section or adding localized metal reinforcements will increase stiffness, as the metal thickness increases, it is more difficult to form the part due to limitations in metal forming machines. Importantly, in many applications increasing metal gauge will not work effectively because stiffness frequency is proportional to section stiffness divided by section mass:
f≈(i.e., frequency is proportional to the square root of stiffness over mass). Mass and stiffness are increased proportionately, with no resultant change in the dynamic stiffness frequency of the part.
In addition, filling a section entirely with foam creates a large heat sink and requires elaborate sealing operations to close access holes in the stampings. Also, the presence of the foam may interfere with the placement of interior trim panels, wiring harnesses, and hinges.
Accordingly, it would be desirable to provide a low-cost technique for increasing the stiffness of a section without proportionately increasing the mass. The present invention provides sections which have increased stiffness values with no significant increase in mass and without the use of high volumes of expensive resins. In many applications, the present invention reduces vibrations which cause unwanted xe2x80x9cshakexe2x80x9d of a component.
In one aspect, the present invention provides a hollow laminate beam characterized by its high-stiffness-to-mass ratio. The beam has an outer portion which is separated from an inner tube by a thin layer of structural foam. The cavity defined by the beam may be open or closed along its length.
In another aspect, the hollow laminate beam of the present invention is an automotive radiator support beam having an outer metal section and a generally rectangular inner tube, which may be open on one side. At least three sides of the rectangular inner tube are coated with a structural foam which is disposed between the rectangular inner tube and the outer metal section and in contact therewith. A metal cap is welded in place to complete the beam and retain the inner tube. The diameter of any through holes in the inner tube which are in alignment with through-holes in the outer portion are larger than the outer portion through-holes such that the structural foam does not block the through-hole clearances of either metal thicknesses.
In still another aspect, the laminate beam of the present invention is an automotive windshield pillar. A hollow metal tube is disposed within the pillar and is separated from the outer pillar stampings by a thin layer of structural foam.
In still another aspect, the laminate beam of the present invention is an automotive rocker panel assembly. The rocker panel assembly comprises mating inner and outer panel sections which form a generally rectangular rocker panel wall structure. Positioned within the rocker panel wall structure is a closely fitting inner metal tube which defines a cavity. A thin layer of structural foam is disposed between the rocker panel wall structure and the inner tube structure.
In still another aspect the beam is a motor vehicle drive shaft. An inner tube is closely received within the outer drive shaft housing, thereby defining an annulus. A layer of foam is disposed in the annulus.
The present invention also provides a method of increasing the stiffness-to-mass ratio of a beam, wherein the beam defines a cavity. The method includes the steps of forming a tube which fits within the cavity defined by the beam; placing a layer of resin on at least a portion of the outer surface of the tube; and inserting the tube in the cavity, with the resin contacting the inner wall of the tube.
In still another aspect, the present invention comprises a C-rail section for use in truck frames. An inner stamped or rolled C-shaped member is separated from the outer C-frame rail by a layer of resin-based material.
In still another aspect, a plurality of plugs made of a foam which disintegrates at high temperatures are used to close through-holes in a part which is subsequently filled with a core material. The part is then passed through an oven which melts or disintegrates the plugs.
In still another aspect, the present invention provides a door beam or side impact beam for a motor vehicle door which provides increased compression resistance at minimal cost and weight. A local reinforcement is provided in the midspan of a steel shell. The local reinforcement includes a high-strength, thin steel stamping and a layer of thermally expandable foam. This allows the outer shell to be formed of relatively inexpensive mild steel. The foam is sandwiched between the outer shell and the thin steel stamping.
In one aspect, the outer steel shell and inner steel stamping having mating flanges that are welded together by spot welds.
In still another aspect, a reinforced bumper is provided for a motor vehicle. A local reinforcement provides a steel-foam-steel laminate high-strength structure.